The present invention relates generally to paper making and more particularly, to the construction of a press section of a paper machine.
The press section is particularly directed to improvements in press sections which include, in the direction of web passage through the press section, two successive, separate dewatering press nips between which the web travels in a closed draw. The first one of the separate press nips is formed between two preferably hollow-faced press rolls. The press section includes a first fabric, which functions as a pick-up fabric onto which the web is transferred from the forming wire and, in total, the press section includes at least three fabrics arranged so that one of the fabrics acts as a press fabric in the first nip and a second fabric acts as a press and/or transfer fabric both in the first nip as well as in the second nip. The web is transferred after the last press nip to the drying section as a closed draw.
Dewatering of a paper web by evaporation consumes large amounts of energy and is therefore a costly and uneconomical operation. For this reason, it is usually attempted to remove as much water as possible from the web by mechanical means before the web reaches the drying section of the paper machine. The last stage where water is mechanically removed from the web in a paper machine is the press section where water is removed by pressing the web between rolls. It is well-known that water will be more readily removed from the web when the temperature of the water is elevated since the viscosity of the water and the springback coefficient or coefficient of compression elasticity of the web are thereby reduced as is the surface tension.
As the rates of production of paper machines increase, one of the major bottle-necks which will limit further increased production rates is the existence of open draws of the web after the press section, either from the press section to the drying section or in the first free spaces in the drying section itself.
The most common conventional arrangement for dewatering fibrous webs, in particular paper and cardboard webs, is one where the web is passed through a press nip formed between two rolls situated in facing relationship to each other.
As is well known, one or two press fabrics are used in such dewatering nips, the fabrics carrying the water drained from the web away from the nip and also functioning to carry the web forwardly after the press nip.
If it is attempted to increase the dewatering capacity of a nip press by increasing the nip pressure, a certain line pressure is eventually reached beyond which any increase in the nip pressure is no longer desirable since the structure of the web and the press fabrics cannot withstand such increased compression pressures.
The dewatering capacity of a nip press can be increased by lengthening the zone of the roll nips through the use of larger diameter rolls and softer press fabrics. However, a limit is again reached using even this technique. Conventional nips which use press-suction rolls are also limited by the common requirement that the press-suction roll have a variable-crown arrangement. However, it is not usually possible to provide a press-suction roll with a variable-crown due to the perforations in the mantle of the suction roll and/or since the interior of the press-suction roll is occupied by suction equipment to the extent that the space available for variable-crown equipment is not insufficient.
Accordingly, for these and other reasons, so-called extended-nip presses have recently been proposed. In this connection, reference is made, by way of example, to U.S. Pat. Nos. 3,808,092; 3,808,096; 3,840,429; 3,970,515; 4,201,624; and 4,229,253, to G.B. Pat. Appl. No. 20 57 027, as well to Finnish Patent Applications Nos. 82-3500; 83-0995; and 83--1028, the Finnish Applications being assigned to the assignee of the instant application.